Polyethylene prepared by the high-pressure process is known as one of polyolefin resins having relatively high transparency, and has been widely used for application in film or hollow container. Speaking about the use of polyethylene in film, however, the high-pressure polyethylene, when molded into film by the air-cooled inflation process often used in common film formation, it becomes difficult to give the product having sufficient transparency, impact resistance and tear resistance. In light of the above-mentioned properties of high-pressure polyethylene, there has been proposed a process for the preparation of improved polyethylene copolymers free from such disadvantages by copolymerization of ethylene and other polymerizable monomers, for example, vinyl acetate This process, however, undesirably brings about such problems that the film resulting from the ethylene copolymer prepared thereby decreases in mechanical strength and rigidity, or said film is liable to undergo blocking, thereby throwing hinderances in the way of the molding operation.
By the way, a copolymer of ethylene and .alpha.-olefin of at least 3 carbon atoms prepared by using a Ziegler catalyst is known as a resin excellent in mechanical strength and having a density nearly equal to that of the high pressure-polyethylene. Generally, ethylene/.alpha.-olefin copolymers prepared by using a vanadium based catalyst as the Ziegler catalyst are low in melting point, hence heat resistance and mechanical strength comes into question.
An ethylene/.alpha.-olefin copolymer prepared by using a mixture of a titanium based catalyst with an organoaluminum compound catalyst component as the Ziegler catalyst, on the other hand, is high in melting point and excellent in heat resistance in comparison with the above-mentioned ethylene/.alpha.-olefin copolymer prepared by using the vanadium based catalyst. However, when this ethylene/.alpha.-olefin copolymer prepared by using the above-mentioned mixture as the Ziegler catalyst contains, propylene of 3 carbon atoms or butene-1 of 4 carbon atoms as the .alpha.-olefin, the mechanical strength of said copolymer comes into question. On that account, the .alpha.-olefin used in this ethylene/.alpha.-olefin copolymer is selected from among .alpha.-olefin having at least 6 carbon atoms in order to obtain sufficient mechanical strength of the resulting copolymer. A film obtained from a copolymer of ethylene and .alpha.-olefin having at least 6 carbon atoms is excellent in impact strength, however, has such problems that because of its tear strength highter than that required, the film does not tear easily and becomes poor in tear properties.
The present inventors have already found that a resin having both excellent impact strength and appropriate tear strength, i.e. excellent tear properties, may be obtained by copolymerizing ethylene and pentene-1 of 5 carbon atoms using a titanium solid catalyst component and an organoaluminum compound catalyst component.
It has been ascertained later on, however, that the resin thus obtained is inferior in melt tension and flow characteristics to the high-pressure low density polyethylene, and depending on the catalyst system used therefor, it becomes difficult to obtain a copolymer having good transparency. In the course of forming a film from a resin, when the resin is poor in flow characteristics and melt tension (hereinafter these properties are sometimes called "moldability" by combining them together) even when its original properties are excellent, the resulting film becomes poor in bubbling stability and the surface of the film is liable to wrinkle, and further it is difficult to thin the film. Similarly, in molding a hollow container, it is difficult to aim at speeding up of the molding operation. In either case, the use of a resin poor in moldability is not desirable. For improving flow characteristics of a resin, usually a melt index of the resin is increased, that is, a molecular weight of the resin is decreased. In this case, however, the resin thus treated becomes poor in melt tension and also decreases in impact strength, though the flow characteristics of the resin are improved.
In order to solve the above-mentioned problems, it is also not impossible to try to improve the polymer in flow characteristics by widering a molecular weight distribution and/or a composition distribution of said polymer. The resin thus treated, however, deteniorates in transparency as well as in impact resistance and becomes also sticky, though it certainly improves in flow characteristics. The resin thus obtained is also found to be poor in melt tension.
Moreover, as the result of investigating the characteristics of the ethylene/pentene-1 copolymer conducted by the present inventors, it has been found that there is still a room for improving blocking resistance and heat sealing properties at low temperature among the chracteristics of the copolymer.